Field of the Invention
The present invention relates to a method for recording MR signals with an image-recording sequence, wherein undesired signal coherence pathways are suppressed. The invention also relates to a magnetic resonance apparatus designed to implement such a method.
Description of the Prior Art
MR imaging has become established as an important diagnosis method in recent decades. For the creation of MR images, a number of different sequences of RF and gradient pulses are used to generate the desired image contrast in each case. A special sequence is the twice-refocused spin echo sequence, also known as double spin echo sequence, that has one RF excitation pulse and two RF refocusing pulses. After each RF pulse, gradients are switched with moments that are, for example, effective for contrast generation or preparation or for spatial encoding.
Twice-refocused spin echoes are used for example for diffusion imaging with the suppression of residual eddy-current fields. A further field of application is spatial localization when slice selection gradients are switched on different axes with each RF pulse. This enables image volumes restricted to one, two or three dimensions to be excited. In addition, twice-refocused spin echoes can be of interest in the presence of inhomogeneous B1 fields since quadratic-phase adiabatic RF pulses can be used for refocusing. Phase dispersion along the slice normals can be cancelled simply by the use of two suitable adiabatic pulses. A further possibility for the use of double spin echoes is in the environment of spectroscopic imaging (PRESS (Point RESolved Spectroscopy)).
One challenge with the use of multiple RF pulses is the high number of coherence pathways that are thereby generated. Unavoidable spatial variations of the B1 field can have different impacts on an RF pulse. For example, each RF pulse that is radiated can act on coherences in the form of excitation, refocusing, storage or restoration. In addition to the desired double spin-echo coherence, the following potentially interfering signal pathways occur with RF pulses: three FID signals (full induction decay), three spin echoes, one stimulated echo and one antistimulated echo.
Numerous methods for suppressing all undesired coherence pathways are known for twice-refocused echoes for diffusion imaging, for example from US 2007/0167732 A1, DE 10 2011 005 084 B3 or DE 10 2009 019 895 A1. In particular, in DE 10 2011 005 084 B3, these disclosures are restricted to a special case involving the use of diffusion-encoding gradients and cannot be transferred to the generalized case of twice-refocused spin echoes. In addition, the only case considered is one in which diffusion gradients are applied during the entire duration of the preparation. Variants with pauses between diffusion gradients, which would be necessary for more flexible diffusion encoding, are not possible.
The known spoiler schemes for spectroscopic localization by means of twice-refocused spin echoes also only resolve the problem for a special case with defined gradient moments on the three axes. In addition, individual signal pathways are only suppressed along one coordinate axis in each case resulting in higher dephasing moments for each axis associated with higher gradient amplitudes and/or longer pulse durations and urgently requiring dephasing on all axes.
A further method for suppressing undesired coherence pathways entails the use of phase cycles. In this case, a measurement is repeated many times with different RF pulse phases and the signals recorded are added complexly. Here, the desired signal pathway is constructively amplified while the undesired coherence pathways are destructively extinguished. However, an approach of this kind is very sensitive to temporal variations and instabilities, caused, for example, by the movement of the person being examined.